This may not be appropriate place to ask the question but I hope to get an answer.

Normally, everyone has a national ID Number e.g. national identification card or passport. This number is normally unique across all the citizens of a country. But then we have other ID numbers e.g. registration number of a student at a university, or ID for health system or driving license etc.. I was wandering if there is any specific reason because of which all these numbers are kept separate ? What will be the repercussions of using the National Identification number in all the systems e.g. health system, driving license ++++?